Implements to be powered by the PTO shaft of a tractor are commonly designed to operate at only one or other of two speeds, namely 540 RPM and 1000 RPM. Different spline geometries are used on the input shafts of different implements to ensure that they are not driven at the incorrect speed.
Some tractors have a PTO output shaft that will only fit one type of implement. However, tractors of sufficient power to drive any type of implement have interchangeable stub shafts connected to the engine driven PTO shaft. The stub shafts have different spline geometries on their output end to allow coupling to either type of implement.
Because the tractor PTO shaft can be driven at different speeds, there is a risk of the speed being set incorrectly for a given implement. For this reason, there is a need to be able to monitor the type of stub shaft mounted on the PTO shaft so that the PTO shaft can only be operated at the speed appropriate for the driven implement.
EP 0 842 804, which is believed to represent the closest prior art to the present invention, discloses a system in which toothed wheels associated with stationary sensors are fitted to the input shaft of the PTO gearbox and to the PTO stub shaft, the toothed wheels of different stub shafts having different numbers of teeth. By measuring the ratio of the pulses produced by the two sensors, it is possible to ascertain the type of stub shaft fitted to the PTO shaft.